Control for Geothermal Heating System

ABSTRACT

A geothermal energy transfer system has a heat transfer loop associated with a heat pump and a heat absorption loop to circulate fluid through an energy source, such as the ground or body of water. The loops are connected through a reservoir and each loop has a circulating pump to circulate fluid through respective loops. The flow rates of the pumps are selected to optimise energy transfer in each loops and differences in the flow rates are absorbed in the reservoir.

FIELD OF THE INVENTION

The present invention relates to geothermal energy transfer systems.

SUMMARY OF THE INVENTION

It is well known to use a heat pump to transfer energy between aconsumer of energy, such as a building, and a source of energy such asthe surrounding environment. The heat pump uses a closed cycle thatpasses a refrigerant through an expansion phase, that requires theabsorption of external energy, and a compression phase, which rejectsenergy to the building. In order to supply energy to a particularlocation, the rejected heat is transferred in to the heating system ofthat location and the energy required to effect the expansion of therefrigerant is absorbed from an external source. Similarly, when heat isto be extracted from the location, the location acts as a source andsupplies the energy for the expansion of the refrigerant and the heatgenerated during compression is rejected to the surrounding environmentthat acts as a consumer.

The environment may be the air itself, as is the Case with traditionalair conditioning units or heat pumps. However, such an arrangement has apoor efficiency due to fluctuations in the air temperature.

A preferred external source has a substantially constant temperature andthe ground or large body of water are typically used. It is thereforeknown to provide a heat exchange loop between the heat pump and such asource so that heat may be absorbed in to the loop to supply energy tothe heat pump or may be rejected from the loop to remove energy from theheat pump. The loops are typically an extensive run of pipe containing asaline, glycol or ethyl alcohol based heat exchange fluid. The pipe isburied in a trench between one or two meters below the normal surface.At that depth, the earth is at a substantially constant temperature andprovides an energy source to either provide energy to or absorb energyfrom the heat transfer fluid because of the temperature differentialbetween the heat exchange fluid and the surrounding.

Where available, a large body of water may be used as the energy source.The heat transfer loop is placed in the water and heat transfer fluidcirculated through the loop.

The heat exchange loop is typically closed to isolate the heat transferfluid from the environment. To compensate for losses of fluid andchanges in the condition of fluid, a flow centre is placed in the heatexchange loop to subdivide the heat exchange loop in to a heat transferloop and a heat absorption loop.

In a non-pressurized system, the flow centre acts, as a reservoir forheat transfer fluid. In a pressurized system, a dedicated reservoir isnot provided as the system is typically charged with air after filling.In both applications, the flow center is usually placed between the loopthat passes fluid through the heat pump (the heat transfer loop) and theloop that passes fluid through the ground or water loop (the heatabsorption loop). A pump circulates the heat transfer fluid through theheat transfer loop and returns it to a manifold from which the heatabsorption loop is supplied.

These arrangements typically size the circulating pump to maintain aturbulent flow through the heat transfer loop. However, such anarrangement has been found to introduce a loss of efficiency in theoverall performance of the energy transfer system.

It is therefore an object of the present invention to obviate ormitigate the above disadvantages.

In general terms, an energy transfer system includes a first loop tocirculate fluid through a heat pump and a second loop to circulate fluidthrough a geothermal energy source. Each of the loops is connected to aflow center to provide a reservoir of fluid for circulation. Arespective pump is connected in each of said loops to establishrespective flow rates of fluid in each of said loops, with balance flowbeing provided by the flow center.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way example onlywith reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of an energy transfer system;

FIG. 2 is a perspective view of a flow center;

FIG. 3 is a schematic representation of flow through the flow center ofFIG. 2;

FIG. 4 is a view, similar to FIG. 2 of an alternative flow center;

FIG. 5 is a schematic representation of flow through the flow center ofFIG. 4;

FIG. 6 is a further embodiment of the energy transfer system;

FIG. 7 is a side elevation of a further embodiment of flow centre;

FIG. 8 is a section on the line VIII-VIII of FIG. 7;

FIG. 9 is a section on the line IX-IX of FIG. 8.

FIG. 10 is a flow chart showing a first control strategy for operationof the heating system of FIG. 1,

FIG. 11 is a flow chart showing a second control strategy for operationof the heating system of FIG. 1 in a heating mode, and

FIG. 12 is a flow chart showing the second control strategy foroperation of the heating system of FIG. 1 in a cooling mode.

DETAILED DESCRIPTION OF THE INVENTION

Referring therefore to FIG. 1, a building 10 has a heating and coolingsystem 12 to distribute heat through the building or to remove heat fromthe building. The heat distribution system may be an air circulatingsystem, or a water circulating system that transfers heat betweendifferent areas of the building and a heat source. The heating andcooling system 12 includes a heat exchanger 14 that cooperates with aheat exchanger 16 to transfer heat between a heat pump 18 and thebuilding 10.

The heat pump 18 is of conventional construction and includes a heatexchanger 20 connected in a refrigerant loop 19 to the heat exchanger 16through a throttle valve 22 and a compressor 24. Expansion of arefrigerant through the throttle valve 22 causes heat to be absorbed into the refrigerant and compression of the refrigerant through the pump24 causes heat to be rejected.

The heat exchangers 16 and 20 absorb or reject the heat depending uponthe mode of the operation of the refrigerant cycle. A reversing valve 23reverses the flow direction to allow the heat pump 18 to function in aheating mode to supply heat to the building, or a cooling mode in whichheat is extracted from the building 10. A thermostat 27 and controller25 is incorporated in to the system 12 to control operation and maintainthe required temperature in the building 10.

The heat exchanger 20 cooperates with a further heat exchanger 26 totransfer heat between the refrigerant loop 19 and a heat transfer loopindicated at 28. The heat transfer loop 28 includes a pump 30 thatcirculates a heat transfer fluid, typically a saline, glycol or ethylalcohol based mixture, through a return pipe 32 and a supply pipe 34.

The pipes 32, 34 are connected in series with a pair of header pipes 36,38, one of which, 36 acts as a supply and the other, 38 acts as areturn. The header pipes 36, 38 that are connected to opposite sides ofa heat transfer unit 40 to provide a heat absorption loop 41. The heattransfer unit 40 may be a loop or multiple loops connected in parallel,to the header pipes 36, 38. The loop is buried in the ground or underwater, or, preferably, is a self contained heat transfer unit of thetype more fully described, in U.S. patent application 61/367,166, andthe contents of which are incorporated herein by reference. The loopsmay also include loops to auxiliary heat consumers, such as a pool orspa, if required and as shown in FIG. 6, with a suffix ‘b’ for clarity.A pump 42 is connected in the header pipe 36 to circulate fluid throughthe heat absorption loop 41 defined by the pipes 36,38 and the heattransfer unit 40.

A flow center 44 is connected in parallel with the pipes 36, 38 and 32,34 through stub pipes 46. The flow center 44 is seen more fully in FIG.2 and, in its simplest form, comprises a cylindrical housing 50 sealedat its lower end. A cap 52 with a vent valve 54 is fitted to the housing50 to provide venting to accommodate expansion and contraction of fluidin the fluid circulation loops 28, 41. The stub pipes 46 are connectedon diametrically opposite sides of the housing 50. In the case of thepressurized configuration, the vent valve 54 is replaced with an airvalve allowing the system to be pressurized. The Cap 52 is installed asto seal the system.

In operation, the heat transfer loop 28 and the heat absorption loop 41are filled with fluid through filling the housing 50. The vent 54 allowsfor venting of air from the system and a cap 52 for adding/replenishingfluid during/after initial installation. The pumps 30 and 42 operate tocirculate fluid through the heat exchanger 26 and through the heatexchanger 40. The pump 30 is sized to provide a turbulent flow throughthe heat transfer loop 28 at a rate that maximizes heat transfer betweenthe heat exchangers 26 and 20. The rate required to attain optimum heattransfer will vary in different design conditions but for a supply offluid at a particular temperature an optimum rate can be determined,from operating characteristics of the heat pump 18.

For a given heat transfer rate into the building 10, and with a designtemperature of the heat transfer fluid and the known characteristics ofthe heat exchanger 26, an appropriate flow rate of the fluid passingthrough the heat exchanger 26 can be determined. Frequently, the designtemperature and flow rates are specified by the manufacturer of the heatpump. For example, with a Geostar Model GT064, a nominal heat transferof 27100 Btu/hr is specified with a flow rate of 16 US gpm and anassumed entering water temperature of 20° F. Correction tables areprovided to compensate for different entry water temperatures (EWT).

Similarly, the pump 42 is sized to provide a circulation through theheat absorption loop at a rate that optimizes the transfer of energybetween the heat exchanger 40 and the surroundings. Again this willdepend upon the particular design conditions but an optimum flow ratecan be attained, taking into account the temperature of the heat source,the thermodynamic properties of the fluid and the heat transfercharacteristics of the heat transfer unit 40.

For the same thermal load, the heat absorption rate from thesurroundings through the heat exchanger 40 may require a different flowrate through the heat absorption loop 41 to that in the heat transferloop, 28.

The pumps 30, 42 can then be sized to provide those respective flowrates. Preferably, each of the pumps 30, 42 are variable flow rate pumpsthat can be adjusted to increase or decrease the flow rate to suitparticular control strategies. Alternatively, one of the pumps 30, 42may be a fixed capacity and the other variable to permit adjustment ofthe respective flow rates. If a steady condition is anticipated thenboth pumps may be of fixed flow rating for the anticipated conditions inthe respective loop. However, as will be explained more fully below, theability to adjust the flow rates may be used advantageously in theoperation and control of the heating and cooling system 12.

As illustrated in FIG. 3, the flow center 44 operates as a reservoir toreceive excess fluid from the heat absorption loop 41 and supply abalancing fluid back into that loop through respective ones of the stubpipes 46. Typically, it is found that the flow rate through the heatabsorption loop 41 is greater than that required in the heat transferloop and so the flow center 44 receives fluid from, and delivers fluidto, the heat absorption loop 41. In FIG. 3, the flow required throughthe heat transfer loop 28 is denoted by Y and the flow rate required inthe heat absorption loop 41 is X+Y. The flow center 44 thus receives Xgallons per minute from the heat absorption loop 41 through one of thestub pipes 46 acting as an inlet and similarly delivers X gallons perminute to that loop 41 from the other stub pipes 56 acting as an outletto supply the pump 42. In one installation with an eighteen kilowattheat load, it has been found that a flow rate through the heatabsorption loop 41 in the order of 23 gallons per minute is optimum witha flow rate through the heat transfer loop 28 of 16 gallons per minute.

Those flow rates will of course depend upon the nature of the heatexchanger 40 and the temperature of the environment T in which the heatexchanger 40 operates.

Fluid circulation in the heat absorption loop 41 may also enable aselective precooling or preheating of the fluid in the flow center 44.For example, when heating a dwelling, the fluid can be preheated in theflow center 44 from fluid circulation in the heat absorption loop 41 andwhen cooling a dwelling, the fluid can be precooled in the flow centerfrom fluid circulation in the heat absorption loop 41.

A further embodiment of flow center is shown in FIGS. 4 and 5 in whichlike components will be denoted with like reference numerals with thesuffix a added for clarity. Referring therefore to FIG. 4, the flowcenter 44 a includes a pair of cylindrical housings 50 a ₁ 50 a ₂. Eachof the housings has a cap 52 a and vent valve 54 a. A balancing tube 60interconnects the upper end of the housings 50 a to allow for fluid toflow between the housing.

Each of the housings 50 a receives the return from one loop and thesupply to another of the loops. Thus, the housing. 50 a ₁ receives fluidreturned from the heat absorption loop 41 a through the pipe 38 a andsupplies fluid to the heat transfer loop 28 a. Similarly, the housing 50a ₂ receives the return through pipe 38 a from the heat transfer loop 28a and supplies fluid through the pipe 34, 36 a to the heat absorptionloop 41 a.

The interconnection of the Units is shown in FIG. 5, from which it willbe appreciated that the differential fluid returned from the heatabsorption loop 41 a through the pipe 38 a may flow from the housing 50a ₁ through the bridge 60 to the housing 50 a ₂ to supplement supply tothe pump 42 a. Again, the pumps 30 a and 42 a will be sized toaccommodate the optimum flow rates through the respective transferloops.

A further embodiment of flow centre is shown in FIGS. 7 through 9 inwhich like reference numerals will be used for like components with asuffix “c” added for clarity. Flow centre 44 c can be usedinterchangeably with the flow centres 44, 44 a, 44 b shown in theprevious embodiments. The flow centre 44 c has a cylindrical housing 50c which is encompassed in an insulating foam 70 and encased in an outercasing 72. A cap 52 c is secured to the housing 50 c and has anupstanding square boss 76. A retaining bracket 78 is fitted over the capand has a square hole 80 that fits around the boss 76. The bracket 78 issecured to the casing 72 by bolts 82 and thereby tamper proofs the capby preventing unauthorized removal. The bracket 78 may also be used,after release of the bolts 82 and inversion of the bracket 78, as awrench to remove the cap 52 c.

A pair of cross tubes 90, 92 extend diametrically through the housing 50c and are sealed at the intersection of the tubes with the housing 50 c.Each end of the tubes 90, 92 is threaded to provide a connection withrespective ones of the pipes 32 c, 34 c, 36 c, 38 c, as will bedescribed in more detail below. Each of the cross tubes 90, 92 has anarray of holes 94 at its midpoint. The holes 94 are evenly distributedaround the circumference of the tube 90, 92 and in the embodiment shownthere are four holes 94 equally spaced about the circumference. Agreater or lesser number of holes 94 may be provided depending upon theparticular circumstances. The aggregated cross section of the holes 94is the same as or slightly greater than the cross section of thecorresponding tube 90, 92.

As can be seen in FIG. 7, a sight glass 96 is provided on the exteriorof the flow centre 44 c to provide an indication of the level of fluidcontained within the flow centre 44 c. Conveniently, a spectrumindicating different colors of fluid corresponding to the approximatecomposition of the solution being circulated through the flow centre isprovided alongside the sight level for easy reference and routinemaintenance.

The tube 90 is connected between the return pipe 38 c of the heatabsorption loop 41 c and the supply pipe 34 c of the heat transfer loop28 c so that one end acts as an inlet from loop 41 c and the other as anoutlet to loop 28. The tube 92 is similarly connected between the returnpipe 32 c of heat transfer loop 28 c and the supply pipe 36 c of theheat absorption loop 41 c to provide respective inlets and outlets.

In operation, fluid from the heat absorption loop 41 c is delivered bythe pump 42 c to the tube 90 where it flows from the return pipe 38 c tothe supply pipe 34 c. Similarly, flow in the heat transfer loop 28 cfrom the pump 30 c is delivered to the tube 92 from the return pipe 32 cto the supply pipe 36 c of the heat absorption loop 41 c. The pumps 30c, 42 c have a differential flow rate so that typically the flowdelivered to the tube 90 from the absorption loop 41 c is greater thanthe flow rate extracted from the tube 90 by the transfer loop 28 c. Thebalance of the flow is discharged through the holes 94 in to thereservoir provided by the interior of the housing 50 c.

Similarly, the flow required from the tube 92 to supply the absorptionloop 41 c is greater than that delivered by the return pipe 32 c of thetransfer loop 28 c and therefore makeup fluid is provided through theholes 94 in the tube 92 from the housing 50 c. The holes 94 thereforeprovide for a cross flow between the heat transfer loop and absorptionloop to maintain the desired flow rates as determined by the respectivepumps.

The effect of the delivery of the fluid in the return pipe 38 c to thetube 90 is to supply it directly to the inlet to the pump 30 c,effectively supercharging the inlet to pump 30 c to a positive pressure,to ensure that it is operating under optimum conditions. The pump 30 cis not required to operate at a reduced inlet suction pressure, but atthe same time ensures that the required flow rates between the two loopsis maintained to provide optimum efficiencies.

The controller 25 is used to control operation of the heating system 10and may be a simple thermostat interacting with the heat pump 18 toswitch pumps 30, 42 on or off. However, as explained in greater detailbelow, the controller 25 may also be used to modulate operation of thepumps 30, 42. The pumps 30, 42 may be fixed flow rate pumps, or one pumpmay be variable and the other fixed. In the preferred implementation,each of the pumps 30, 42 is a variable flow pump to provide differingflow rates in the respective loops 28, 41. An example of such a pump anda suitable controller is a Danfoss VLT micro drive—FC51. The controller25 provides a variable reference frequency to the motor of the pumpwhich adjusts the rotational speed of the motor to match the referencefrequency. Variable flow rates may also be provided by using a pair ofpumps connected in series and selectively switching one of the pumps onor off.

The controller 25 in a preferred embodiment, is a programmablecontroller having outputs, namely Y₁, Y₂, and O. Outputs Y₁, Y₂ controloperation of the compressor 24 with Y₁ calling for a first intermediateload, typically 67% of compressor capacity, and Y₂ calling for a full,100% load. The output O controls reversing valve 23 to switch betweenheating mode and cooling mode.

In general terms, the output Y₁ is used to provide a reference frequencythat sets the pump 30 at an intermediate flow rate, to match therequired flow rate through the loop 28 when the compressor 24 has anintermediate load, and to maintain the pump 42 at a correspondingpredetermined flow rate in excess of pump 32. Upon an output Y₂ beingreceived, when the compressor is conditioned to full load, the output ofeach pump 30, 42 is correspondingly increased to match the flow rates tothe full load operating condition of the system.

The flow centre 44 c of FIG. 9 facilitates initial setup of the relativeflow rates in the heating and cooling system 12, which, in turn,enhances control of the system 12 after the initial setup.

During initial setup, assuming a single, variable flow pump 30 c is usedin the heat transfer loop 28 c, the pump 30 c is set to an'initialintermediate flow rate, typically that specified by the manufacturer ofthe heat pump 18. The flow rate is determined by measuring the pressuredrop across the heat exchanger 26 c, after applying a correction factorto accommodate for varying temperatures of the fluid in the loop 28 c. Afirst set point x₁ of the reference frequency is established for therequired flow rate of pump 30 c. With the flow rate in loop 28 cestablished, the flow rate of the pump 42 c is adjusted to match that ofthe pump 30 c. This is facilitated in the flow centre 44 c by reducingthe level of fluid through the drain port provided on the sight glass,so that the fluid is level with the upper cross tube 90. At this level,the relative flow rates in the loops 28 c, 41 c, can be observed fromthe flow through the cross ports 94. When the flows are equal, there isno net flow across the ports 94 and the flow rates are balanced.

Upon attaining a balanced flow, the pump 42 c is adjusted to increasethe flow in the loop 41 c to achieve a nominally increased flow rate. Ithas been found that an increased flow rate of 5%-10% is satisfactory fortypical installations. A first set point z₁ of the reference frequencyis established for the pump 42 c.

The demands of the heat pump 18 with the compressor 24 operating at fullload require an increased flow rate in the loop 28 c. Accordingly, asecond set point, x₂, is established for the increased flow raterequired from pump 30 c, either empirically or by measuring the pressuredrop across the heat exchanger 26 c as specified for a full load, and acorresponding set point z₂ established for the pump 42 c. This may bedone by observing net flows in the flow centre 44 or by extrapolationfrom the previous settings.

With the initial conditions established, the fluid is replaced in theflow centre 44. The controller 25 may then be used to control the pumps30, 42 in normal use.

In one embodiment of the control strategy, as shown in FIG. 10, theoutputs of controller 25 are used to adjust the flow rates from thepumps 30 c, 42 c, in the required ratio, to meet the demands of thesystem 12.

The output O determines the mode, heating or cooling, and upon thethermostat calling for an increase in temperature (in the heating mode),or a reduction of temperature (in the cooling mode), an output Y₁ isapplied to the compressor 24 and each of the pumps 30, 42.

The compressor 24 operates at the intermediate load (e.g. 67%) and thepumps 30, 42 circulate fluid at the rates determined by the set pointsx₁, z₁ respectively.

If after a set period, 30 minutes to 120 minutes, the thermostat has notattained its required temperature, or if the thermostat calls for animmediate increase in temperature greater than 2° C., the controller 25provides outputs Y₂ to the compressor 24 and each of the pumps 30 c, 42c.

The compressor 24 increases to full load and the output of pumps 30 c,42 c, is increased to set points x₂, z₂ respectively. The system 12operates at these conditions until the required temperature is reached,or a further time limit is reached and the auxiliary heat is switched onby output W.

Upon attainment of the required temperature, the controller 25 removesthe outputs Y₁, Y₂ and W, and the system returns to an at restcondition, with the compressor and the pumps 30 c, 42 c switched off.

By matching the flow rates of the pumps 30 c, 42 c, to the demands ofthe compressor, the operation of the overall system may be optimizedwith the flow rates in the respective loops maintained in the requiredratio.

It will be appreciated that the relative flow rates of the pumps 30 c,42 c may be adjusted to suit a particular installation and systemconfiguration with the set points for each pump chosen to provide theoptimum flow rates.

The flexibility provided by the controller 25 and the use of a pump ineach of the loops 28 c, 41 c, may be utilized to further optimize theoperation of the system 12.

As shown in the schematic of FIGS. 11 and 12, different operatingconditions are attained depending on the mode of operation.

In a heating mode, i.e. one in which heat is transferred in to thebuilding 10, the set points z₁, z₂ are selected so that the relativeflow rate of the pump 42 c is increased beyond that needed to balancethe flows in each loop. Typically a flow rate of 110% of that of thepump 30 c is found satisfactory, although flows in the range 105% to125% may be used. The increased flow from pump 42 c is transferredthrough the flow centre 44 c between the cross tubes 90, 92, and is usedto heat the fluid returning from the loop 28 c as it enters the loop 41.

The fluid delivered through loop 41 c to the loop 28 c will have atemperature approaching that of the ground source.

The fluid is transferred at that temperature to the inlet 34 c of theloop 28 c and delivered to the heat exchanger 26 c. Heat is extractedfrom the fluid for delivery to the building, resulting in a significantreduction of the temperature of the fluid. The fluid is returned at thattemperature to the flow centre 44 c, where it is delivered through thecross tube 92 to the supply header 36 c.

Because of the reduced temperature, there is a risk, in some operatingconditions, that localized freezing may occur, particularly on thesurface of the header 36 c and heat exchanger 40 c, that impairs heattransfer. This is mitigated by the admixture of the excess flow from thepump 42 c with the return flow from the loop 28, which elevates thetemperature of the fluid in the loop 41 c. With a flow rate of the pump42 at 110% of pump 32, and a fluid temperature at around −5° C., it hasbeen found that the overflow and admixture can elevate the fluidtemperature by 2° C., sufficient to mitigate the surface freezing.

When operating in a cooling mode, i.e. when heat is rejected to theground source, as shown in FIG. 12, the temperature of returning fluidis elevated. In this situation, admixture with the excess flow willreduce the temperature and reduce the rate of dissipation across theheat exchanger, which is undesirable. Accordingly, the pump 42 iscontrolled so that the flow differential is reduced and pump 42 is setto operate at a slightly greater flow rate, i.e. 2% greater than pump32. In this case, the set points z₁, z₂ are selected to minimize thecross over flow in the flow centre.

The control strategy, therefore, operates the pump 42 c to over supplythe loop 28 c during heating mode to permit admixture, whereas incooling mode the admixture is minimized by matching the outputs of pumps42 c and 28 c.

During transient conditions, the variability of the flow rates may alsobe used to advantage and coordinated with the operation of the heat pump18, as also shown in the schematics of FIGS. 11 and 12.

Assuming the building 10 is at the required temperature, the controller25 maintains the heat pump 18 inactive and both pumps 32, 42 off, i.e.no flow.

When the controller 25 calls for heating, initially the fan and heatpump compressor associated with the building 10 is switched on asindicated as “C on at L1”. After a pre set delay, e.g. 5 seconds, acontrol signal Y₁ is sent to the pump 30 c to initiate flow in the loop28 c at the rate determined by the set point x₁. After a further delay.e.g. 20 seconds, the control signal Y₂ is applied to the pump 42 tooperate it at its maximum flow rate, i.e. at set point z₂ and the pump30 is maintained operating at the intermediate speed x₁. The increasedflow rate is accommodated in the flow centre 41 and is maintained for aninitial purge period, typically a period sufficient to provide acomplete circulation of fluid in the loop 41, in the order of 300seconds. A flow ramp up period of 30 seconds is provided to avoid suddenchanges. If preferred, a higher flow rate than the set point z₂ may beused for purging, but it is convenient to use the set point 22.

After the initial purge period, the control signal to the pump 42reverts to Y₁ and the output of the pump 42 will be ramped down over aperiod of 30 seconds, to set point z₁. The pump 30 is switched on at setpoint x₁. as the heating mode is selected, the set point z₁ provides anover capacity providing admixture with fluid returning from loop 28.

If the required temperature is attained, the control signal Y₁ isremoved. The controller 25 asserts an output Y₂ to the pump 42 for ashut down period, typically 300 seconds, to maintain circulation in theheat transfer loop 41. Thereafter, the flow rate is ramped down and thepump 42 switched off.

When the required temperature has not been attained after a presetinterval, i.e. 30-120 minutes, the output Y₂ is asserted to each of thepumps 32, 42 and both operate at their maximum respective rates, asdetermined by set points x₂ and z₂. Once the temperature is attained,the pumps 32, 42 are shut down as described above.

A similar sequence is implemented in the cooling mode, with the setpoint z₂ of pump 42 being the lower value that matches the maximumcapacity of the pump 32.

The independent operation of the two pumps 32, 42, may therefore be usedto establish optimum flow rates in each loop for steady state andtransient conditions, without impacting on the design conditions for theheat pump 18.

It will be appreciated that the examples above are exemplary and othercombinations may be used to meet the particular design parameters of thesystem 12.

1. A geothermal energy transfer system to transfer thermal energy between an energy source and an energy consumer, said system comprising a first loop to circulate heat transfer fluid through said source, a second loop to circulate heat transfer fluid through said consumer, a fluid reservoir connected to each of said loops to receive fluid from and deliver fluid to each of said loops, a first pump to circulate fluid in said first loop and a second pump to circulate fluid in said second loop.
 2. The system of claim 1 wherein at least one of said pumps has a variable flow rate.
 3. The system of claim 2 wherein both of said pumps have a variable flow rate.
 4. The system of claim 2 including a controller to control the flow rate of said pumps.
 5. The system of claim 4 wherein said controller controls a heat pump thermally connected to one of said loops and, said flow rates are coordinated with the operation of said heat pump.
 6. The system of claim 1 wherein each of said loops has a supply and a return and the supply of one of said loops is connected to the return of the other said loops.
 7. The system of claim 6 wherein said reservoir is connected between the supply and returns of each loop to accommodate differing flow rates therein.
 8. The system of claim 7 wherein the supply and returns are connected to respective inlets and outlets of said reservoir.
 9. The system of claim 8 wherein said reservoir has a pair of inlets and a pair of outlets and said pumps are connected to respective pairs of said inlets and outlets.
 10. The system of claim 9 wherein an inlet connected to one of said loops is connected to an outlet connected to the other of said loops.
 11. The system of claim 10 wherein each of said inlets and outlets is in communication with said reservoir.
 12. The system of claim 1 wherein the flow rates of said first and second pumps are different and said reservoir accommodates the differential in flow.
 13. The system of claim 12 wherein at least one of said first and second pumps is adjustable for flow rate.
 14. The system of claim 13 wherein both of said pumps are adjustable for flow rate.
 15. The system of claim 14 wherein operation of said first and second pumps is controlled by a controller.
 16. The system of claim 15 wherein said controller adjusts said first and second pumps between a first condition in which both pumps have an intermediate flow rate and a second condition in which both pumps have a flow rate greater than said intermediate flow rate.
 17. The system of claim 16 wherein said flow rates are maintained in a predetermined ratio in both said first and second conditions.
 18. The system of claim 17 wherein said rates may be varied depending on the operational mode of said energy transfer system.
 19. A flow centre for use in a geothermal energy transfer system, said flow centre comprising a reservoir to contain fluid, a first inlet for connection to a return of one loop and a supply of another to receive a differential flow through said loops, and an outlet for connection to a supply of said one loop and a return of said other loop to supply fluid to make up for a difference in flows in said loops.
 20. A flow centre according to claim 19 wherein a pair of tubes extend through said reservoir to permit connection at opposite ends of said respective supply and return, said tubes having an aperture therein to provide respective ones of inlet and said outlet.
 21. A flow centre according to claim 19 wherein a pair of reservoirs are provided and a conduit is provided to transfer fluid between said reservoirs.
 22. A flow centre comprising a body defining a reservoir, a pair of tubes extending through said reservoir and having opposite ends for connection to respective pipes, and an aperture intermediate said ends to allow fluid communication between said tube and said reservoir.
 23. The flow centre to claim 22 where said aperture is provided by a plurality of holes disposed about the circumference of said tube. 